This invention relates to an extrusion die for the production of a flat sheet of thermoplastic material, the die comprising a pair of die bodies having means together forming a slit along the width thereof through which thermoplastic material is forced to form a sheet or a web, one of the die bodies having a lip adjustable relative to the other of the die bodies for varying the size of the slit opening.
Thermoplastic material, for example granulated polyolefins, are melted and extruded at working temperatures of from 200.degree. to 320.degree. C. The material is forced through the slit of the aforedescribed extension die to form the sheet or web. An instrument for measuring temperature is mounted in the direction of feed of the sheets or webs for measuring the predetermined slit width which is controlled via a computer. Such temperature measuring instruments usually operate according to the reflection principal with ultrared light, or according to the transmission principal with radioactive radiation for continuously measuring the thickness transverse to the direction of feed.
U.S. Pat. No. 3,940,221 discloses an extrusion die generally of the type described above. One die body has a rigid lip and the other die body has a flexible lip forming together therewith a slit through which flowable material is extruded to form a sheet. Such other die body has a recess formed by a pair of spaced shoulders for the reception of a heating and cooling block. The block has a continuous bore coaxial with bores extending through the shoulders, the shoulder bore adjacent the flexible lip being smooth, and the other shoulder bore being internally threaded. An elongated metal rod extends through these coaxial bores and bears at its operating end against the flexible lip. The rod is externally threaded at its other end for threaded engagement with the threaded shoulder bore, the shoulder, providing a stop face for the head of the metal rod. The heating and cooling block has a heating cartridge and a cooling channel through which a gaseous medium flows. Once an initial adjustment has been made by a mechanical turning of the head of the rod, the rod is then adapted for precise and automatic continuous thermal adjustment of the flexible lip. When the thickness of the sheet being extruded is excessive, the heating cartridge is energized to emit heat which is conveyed by conduction to the elongated rod, thereby raising its temperature and causing it to expand. Since the upper end of the rod is threaded to the outer shoulder and thus fixed while the lower end is free to move, the lower end of the rod is forceably displaced in an axial direction, pressing upon the flexible lip and bending it slightly for swinging movement about its necked down portion to narrow the slit. However, since the metal rod operates only under pressure against the flexible lip, only a fine adjustment of the size of the slit is made possible since adjustment is restricted by thermal changes in the length of the rod.
Since initial adjustment is made by a mechanical turning of the head of the rod, the entire width of the thickness of the sheet is determined, and such thickness is thereby controlled or regulated. Since the thermal changes in length of the metal rod is a fraction of a millimeter, the sheet thickness can be controlled only to a very slight extent such that it becomes time consuming to make necessary mechanical readjustments of the rod for producing substantially thicker or thiner sheets to be extruded.
German Pat. No. 3,427,912 likewise discloses an extrusion die for the production of a flat sheet of thermoplastic material. One die body has a bore for the reception of a ram which bears against the flexible lip of the one body and which can be actuated by an eccentric adjustable drive for effecting coarse adjustments of the width of the die slit. For making fine adjustments of the die slit, the other die body has a field translator, for example a piezoelectric translator, which bears against the flexible lip of the other die body and effects changes in length as a function of changes in voltage applied to thereby adjust its flexible lip. The initial signals of this instrument for measuring thickness of the extruded sheets serves as a control signal for the voltage supplied to the translator. The result is a complete separation between fine adjustment and coarse adjustment without the need to rely on thread bolts and the like, which do not permit a setting free of tolerances as this patent indicates. Apparently, this setting is the prerequisite for the accurate control of the width of the die slit. If a change in the basic setting is necessary during extrusion of the sheets or webs, considerable experience and tactile sensitivity is required of the operators to make the necessary coarse and fine settings compatable with one another so that thereafter the fine setting overlaps the region of the slit width to be regulated or controlled.
U.S. Pat. No. 2,387,718 discloses an extrusion die in which each die body has a bore for the reception of a differential adjusting screw which at one end bears against a confronting flexible lip. The lips together define an extrusion slit and together the lips are T-shaped in cross-section. Thus, coarse and fine adjustment of the slit width requires experience and tactile sensitivity on the part of the operator. Not only pressure but also tensile strength must be exerted via the differential screws on the associated lips for effecting coarse adjustment of the slit. Coarse adjustment is effected for operation of an outer coarse screw thread in engagement with a sleeve located in the bore and mounted against axial displacement. Fine adjustment is effected by operation of a fine threaded screw extending from each inner end of the coarse screw element. In addition to the aforementioned requisite of experience and tactile sensitivity required by the operator in effecting adjustment with this arrangement, the setting adjustment is not free from play and, therefore, the setting is not precise since the reaction force corresponding to the pull and pressure loading on the lip must be picked up by the screw threads which forceably exhibit a tolerance.
It can be therefore seen that slit width of known extrusion dies can be coarsely adjusted only with considerable effort but not without free play, and that fine adjustment can be carried out only over a very small control region but likewise not without free play.